Metallized ceramic coating composition



Aug. 26, 1952 L. A. BAIN, JR., ET AL METALLIZED CERAMIC COATINGCOMPOSITION Filed March 22, 1950 INVENTOR5 ATTORNEY Patented ug. i26,11952 METALLIZED CERAMIC ooA'rING `COMPOSITION Lewis A. Bain, `Jr.,Berwyn, and Herbert E. Malone, Riverside, Ill., assignors Ato WesternElectric Company, Incorporated, New York, N. Y., a corporation of NewYork Application March 22, 1950, Serial No. 151,196

(Cl. Z50-22) Claims. l

This invention relates to a coating composil tion for forming aconductive coating on a dielectric base, and more particularly to acomposition for forming a metallic coating on a dielectric base such ason mica, ceramic or crystalline bases.

In the manufacture of condensers forelectrical apparatus, sheets of micacoated with a metallc composition may be assembled in stacks in suchmanner that the mica serves as a dielectric between the coatings, which,in turn, serve as the plates of the condenser. In the past, numerousmethods have `been devised for coating micafor usein such condensers,but difficulty has been` experienced in the devising of proper solutionsto use as coating materials since materials which may readily be`sprayed upon the mica deteriorate on aging, do not form suiiicientlyrugged coatings that will satisfactorily withstand handling duringprocessing, and do not form extremely thin films with adequateelectrical conductivity.

It is an object of the present invention to provide a silver coatingv`composition which may readily be applied to dielectric sheets toprovide thin even metallic coatings on the sheets.

Another object is to provide a composition which will air dry rapidly toa tough lm, which will give complete coverage of the coated object witha minimum of silver, and which will provide a coating having goodelectrical conductivity.

In accordance with one embodiment of the invention, a compositionsuitable for spray coating mica laminations is provided which comprisessilver flake and lead-boro-silicate in a suitable vehicle, which willrapidly dry in air and will burn off when the laminations are latersubjected to a temperature sufficient to fuse the leadboro-silicate andform a metallic conducting coating on the mica, which coating consistspredominantly of silver bound together by the glass formed from meltingthe lead-boro-silicate. The preferredV form of vehicle comprisesan oilmodiiied alkydresin in a solvent to which has been added a dispersionagent.

Another species of the invention is described and claimed in aco-pending applicationby the same inventors, Serial No. 610,844, filedAugust 14, 1945, which application issued May 2, ,1950, as United StatesPatent No. 2,506,130. In the prior application a dispersing agent forthe silver flakes is formed in situ by a reaction between a stearic acidcoating on the silver flakes and triethanolamine to form an organicsolvent soluble soap, which soap is an emulsifying agent. Thisemulsifying agent aids in causing any agglomerates of the silver flakesto break up easily and distribute themselves uniformly over the surfacesof the mica sheets. The present application is a continuation-impart ofthe prior apsheet 'l at the backof the assemblage.

plication and deals with the formation of a conductive coating on adielectric base without the use of an emulsifying agent. i

A complete understanding of the invention may be hadby reference to thefollowing detailed description'when considered in conjunction with theaccompanyingdrawing wherein there is illustrated the method of coatingportions of the surfaces of the mica sheets, and wherein:

Fig. 1 is a face view of a plurality of sheets of mica assembled betweencooperating strips of metallic masking material prior to the spraying ofthe sheets with the silver composition;

Fig. 2 is a vertical sectional view taken substantially alongthe line 22 ofV Fig. 1 Ain the direction of thearrows;

Fig. 3 is a plan view of one of the mica sheets after it has been coatedwith the silver composition; and

Fig. 4 is a sectional View taken substantially along the line 4 4 ofFig. 3 in the direction of the arrows showing the layers of silver onthe mica sheet.

In the drawings, the thickness ofthe various elements has beenmaterially exaggerated to facilitate the illustration of. these parts.For example, the mica sheets which are to be coated are usuallyapproximatelyA .0017' thick.

In the process of manufacturing these condenser elements, aplurality ofsheets or lamienations 'l--l of mica are positioned between cop permasking strips 8-8 and 9--9, on which there has been deposited a mineralwax which acts to nx the mica sheets 1 in position with their edgesmasked under thewcopper tapes or strips 3 and 9.` The upper'mica sheet 1of the assemblage has a strip of masking material l0 along its upperedge'and the succeeding sheets are overlapped by the sheet next adjacentthem to mask the front upper edge of each sheet of mica, whereas thelower rear edge of each sheet of mica will be masked by the nextadjacent lower The bottom of the assemblage may be masked similarly tothe upper end, except that the masking material I0 is placed at` therear of the assemblage, as shown in Fig. l, the right hand side as shownin Fig. 2. After the mica sheets 'l have` been assembled in this manner,a suitable silver composition may` be sprayed thereon `by directing aspray thereof in the directions indicated by the arrows il and l2,whereby there will be no` tendency for4 the composition, Vwhen it issprayed on the mica sheets 1, t0 get under the masking sections of themica sheets or the masking tapes.

The silver composition comprising the preferred embodiment of thepresent invention is composed of.. a silver flake material mixed withlead-boro-silicate fand carried inga fast' drying vehicle whichA willair dry in a short period of time and which, upon being slowly heated toa temperature suiciently high to fuse the leadboro-silicate, will becompletely volatilized out of the composition without igniting.

The preferred vehicle comprises a drying oil modied type alkyd resin,such as a linseed oil modiiied alkyd resin, in a suitable solvent. Oilmodified alkyd resins have been found to be superior to other binders,such as nitro-cellulose or v phenolic resins, because such alkyl resinsmay be eliminated completely during :a iii-ing operation withoutvolatilizing so rapidly that blisters would be raised, which blisterswould tend to separate the coating from the mica base.

Asuitable solvent for the oil modified alkyd resin is a mixture ofxylene and toluene. Other equivalent solvent mixtures may be devised. Ithas been found that the material known commercially as Enn Jay No. 1which is a petroleum Vderived hydrocarbon solvent with` a boiling rangebetween 135 C. and 177 C. and composed mostly of aromatic compounds, isa suitable toluene equivalent and lthat solvent naphtha is a suitablexylene equivalent.

The alkydfresin, being a fast-drying oil modin fied type such as alinseed oil modiiied resin or its equivalent, polymerizes readily,particularly in the presence of silver, which is a catalyst.

Therefore, it has been found advisable to add butyl alcohol to thesolution to lact as a solvent and to retard the polymerization'of ltheresin. it appears that butyl alcohol reacts with the resin components toform a soluble resin.

The silver ordinarily available comprises metallic silver flakes havinga surface coating of approximately 1.5% by weight of stearic acid. Thismay be prepared by ballmilling stearic acid and silver particlestogether with a lmineral spirits solvent'to form silver flakes coatedwith the stearic acid, vwhereupon the composition may be allowed to dryto remove the solvent. Such flakes of silver maybe approximately onemicron thick and fty microns in diameter.

A preferred `form of lead-boro-silicate fis the ternary compound'PbO.SiO2.B2O3. This compound forms a very low melting point glass whichfuses onl the mica sheets without damage to them, and which Vhas acoefficient of expansion similar to that of the mica. Y

The silver ilake Vand lead-boro-silicate in'powder form may be mixedwith the other ingredients of the composition in the following pro- Inthe mixing of these ingredients, the alkyd resin, lead-boro-silicate andsilver ake are iirst mixed in a ball mill together with the butyl alco*hol and solvent naphtha Vwith approximately one-third ofthe toluene and,after these have been thoroughly mixed, the balance of the toluene isadded to the mixture and mixed therewith.

After the mixture has been prepared, it may be sprayed upon the sheetsof mica 'I assembled in the masking strips 8 and 9 by directing thespray in the directions shown by the arrows II and I2, and, afterallowing the coating material to air dry for approximately ve minutes,the assembled sheets and masking strips may either be passed through aconveyor type furnace or placed in a box type furnace and raised to atemperature of 1000 F. to 1100L7 F. and maintained at that temperaturefor approximately two to five minutes. In the air drying of the coating,`the solvents volatilize and in the iiring operation, the alkyd resinvolatilizes out prior to the melting of the lead-boro-silicate. Sincethe mineral Wax vand alkyd resins are not misci ble, the silvercomposition will be deposited over the entire exposed area of the micasheets, but will not tend to seep under the masking strips. The mineralwax, of course, will be burned or volatilized out in the furnace, and assoon the coated mica sheets cool to a temperature below the meltingpoint of the glass, the assemblages of sheets and masking strips may beremoved rom the furnace and the masking strips Vmay be pulled from themica, thus providing condenser members as shown in Figs.'3 and 4, Where--in the Vsilver Vcoatings I3 and I4 extend to oppo- `sisting of Percentby "weight Silver particles 40 to 60 Lead borosilicate e 2 to 5 `Cilmodiiied alkyd resin 1 4 to 15 Solvent Yfor the valkyd resin 54 to 19 2.A composition for forming an electrically conductive coating ondielectric sheets .con-

sistingof Approximate Percent by Weight Silver `particles 45.0 `Leadborosilicate rm--- 2.3 Oil modied alkyd resin 4.5

Solvent for the alkyd resin M 48.2

3. A composition for Vforming `an 'electrically conductive coating ondielectric sheets consisting of:

Percent 'by weight vSilver .particles s 40 to 69 Lead borosilicate 12 to6 Oil modied alkyd resin c to 15 Butyl alcohol 3 to 6 Solvent for thealkyd resin.` r5l to 13 Ak composition for lforming an electricallyconductive rcoating on dielectric lsheets consisting of:

Approximate percent by Weight Silver particles 45.0 Lead borosilicate2.3 Oil modified alkyd resin 4.5 Butyl alcohol 3.4 Solvent for the alkydresin 44.8

5. A composition for forming an electrically conductive coating ondielectric sheets consisting of:

Percent by weight Silver flakes 40 to 60 Lead borosilicate 2 to 6 Oilmodied alkyd resin 4 to 15 Butyl alcohol 3 to 6 Xylene 17 to 5 Toluene34 to 8 6. A composition for. forming an electrically conductive coatingon dielectric sheets consisting of Approximate percent by weight SilverIiakes 45.0 Lead borosilicate 2.3 Oil modied alkyd resin 4.5 Butylalcohol 3.4 Xylene 16.3 Toluene 28.5

7. A composition for forming an electrically conductive coating ondielectric sheets consisting of:

Percent by weight Silver akes 40 to 60 Lead borosilicate. W meltingpoint 2 to 6 Drying oil modified alkyd resin 4 to 15 Butyl alcohol 3 to6 Solvent selected from the group consisting of Xylene and solventnaphtha 17 to 5 Solvent selected from the group consisting of tolueneand petroleum derived aromatic hydrocarbon with a boiling range between135 C. and

177 C. 34 to 8 8. The composition of claim 7 in which the drying oilmodified alkyd resin is a linseed oil modied alkyd resin.

9. A composition for forming an electrically conductive coating ondielectric sheets consisting of:

Percent by weight Silver flakes 40 to 60 Lead borosilicate, low meltingpoint 2 to 6 Linseed oil modiecl alkyd resin 4 to 15 Butyl alcohol 3 to6 Solvent selected from the group consisting of Xylene and solventnaphtha 17 to 5 Solvent selected from the group consisting of tolueneand petroleum derived aromatic hydrocarbon with a boiling range betweenVi" C. and 177 C. 34 to 8 REFERENCES CITED The following references areof record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,385,580 Knox Sept. 25, 19452,461,878 Christensen et al. Feb. 15, 1949 2,506,130 Bain et al May 2,1950 OTHER REFERENCES Mattiello. Protective and Decorative Coatings,John Wiley and Sons, N. Y., 1942, vol. 2, page 587.

1. A COMPOSITION FOR FORMING AN ELECTRICALLY CONDUCTIVE COATING ONDIELECTRIC SHEETS CONSISTING OF: